The Physics of the Atomic Bomb (Level 4)

Level 4: Nuclear Fusion

The binding energy of a nucleus is the energy needed to separate a nucleus into its respective individual nucleons and is proportional to the stability of the nucleus. For atoms lighter than Nickel-62, binding energy and also the stability of the nucleus increases as atomic mass increases. Energy given off by hydrogen bombs arise from nuclear fusion in which lighter isotopes such as deuterium and tritium ‘combine’ into a much stable element such as helium. The energy released during nuclear fusion is the difference between the binding energies of the isotope used in the reaction and the fission products. The energy released can be calculated using Einstein’s mass-energy equivalence E = Δmc², where Δm is the difference in mass between the start and end nucleus and c is the speed of light (3 x 10⁸ m/s).

However, nuclei are positively charged due to the presence of positively charged protons. Extremely high temperatures are required for the positively charged nuclei to overcome their mutual electrostatic repulsion and gain enough kinetic energy to fuse.

This diagram compares examples of nuclear fission and nuclear fusion.

The Physics of the Atomic Bomb (Level 3)

Level 3: Nuclear Fission

The binding energy of a nucleus is the energy needed to separate a nucleus into its respective individual nucleons and is proportional to the stability of the nucleus. For atoms heavier than Nickel-62, binding energy and also the stability of the nucleus decreases as atomic mass increases. Energy given off by atomic bombs arise from nuclear fission in which heavier isotopes such as uranium or plutonium split into more tightly bound stable elements. In the example of Atomic bombs using Uranium-235 as the fissile material, free neutrons may hit the uranium, resulting in the formation of the highly unstable Uranium-236. It then splits into the highly radioactive but more stable fission products of Barium-144, Krypton-89 and 3 neutrons which collide with more uranium to cause a chain reaction. The energy released during nuclear fission is the difference between the binding energies of the isotope used in the reaction and the fission products.

Chain reaction refers to self-sustaining nuclear fission. As seen from the example of atomic bombs using Uranium-235, neutrons produce in a fission reaction go on to trigger more fission reactions. The critical mass is the amount of fissile material needed to sustain an exponentially growing chain reaction. What this means is illustrated in the diagram below in which the number of nuclei undergoing fission reactions keeps increasing. However, do note that not all neutrons produced in fission reactions will trigger additional fission reactions.

© BBC Bitesize

The Physics of the Atomic Bomb (Level 2)

Level 2: Nuclear Emissions and their Penetrating Powers

As mentioned in Level 1 of Physics, radioactive decay is the process in which unstable nuclei try to become more stable by ejecting particles or energy. There are 3 ways in which radioactive decay can occur. 

Radiation can be absorbed by objects in its path and becomes less intense the further they are from the radioactive material. This is because the nuclear emissions become more spread out. The thicker the substance, the greater the amount of radiation absorbed.

1. Alpha Particles

When an alpha particle is ejected from an unstable nucleus, the nucleus loses 2 protons and 2 neutrons. Alpha particles have the largest mass among the 3 types of nuclear emissions. They collide with O₂ and N₂ molecules in the air, losing some of their energy in ionizing the air molecules until eventually they give up all of their energy and are absorbed. [Ionization is the process in which an atom or molecule loses or gains electrons. Since electrons have a relative charge of -1, the atom or molecule can become Ions which are positively or negatively charged particles.] Hence, alpha particles have poor penetrating power, and can be absorbed by a thin sheet of paper or a few centimeters of air. They are the slowest type of nuclear emission and have a speed 1/10 of the speed of light (3x10⁸ m/s)

2. Beta Particles

In beta decay, the unstable nucleus converts a neutron into a proton and an electron. The beta particle is an electron that is ejected from the nucleus at a speed 9/10 of the speed of light. Beta particles carry less charge (-1) compared to alpha particles (+2), and thus react less with the atoms and molecules of objects in its path. Beta particles can be stopped by a thin sheet of aluminium.

3. Gamma Rays

After emitting an alpha or beta particle, the remaining nuclei may be at a higher energy level. When it returns to its 'normal' state, gamma radiation is emitted in the form of gamma rays. Gamma rays travel at the speed of light, are the most penetrating and can only be stopped by many centimetres of lead, or many metres of concrete.

Content adapted from: 

http://www.bbc.co.uk/schools/gcsebitesize/science/21c_pre_2011/energy/nuclearradiationrev2.shtml

Q: Suppose you are given 3 radioactive cookies: The 1st is an alpha emitter, the 2nd is a beta emitter and the 3rd is a gamma emitter. You must eat one, hold one in your hand, and put the third in your pocket. What can you do to minimize your exposure to radiation?

Ans: Eat the gamma emitter, hold the alpha emitter in your hand, and put the beta emitter in your pocket. 

Q: What is the significance of knowing about the types of nuclear emissions?

Ans: As you will learn in the biology section, different types of nuclear emissions have different penetrating powers and thus have different health effects.

The Physics of the Atomic Bomb (Level 1)

Level 1: Half-life

Radioactive decay is the process in which unstable nuclei try to become more stable by ejecting particles or energy. Radioactivity is directly proportional to the number of undecayed radioactive nuclei present. Since the number of undecayed nuclei of a sample is proportional to the mass of the sample, the radioactive half-life of a given radioisotope (radioactive isotopes of an element) is the time needed for half of the radioactive nucleus in any sample to undergo radioactive decay. After two half-lives, there will be one-fourth the original sample, so on and so forth. 

Q: Do all radioisotopes have the SAME half-life?

Ans: NO. Polonium-215 has a half-life of 0.0018 seconds while Uranium-235 has a half-life of 4.5 billion years.

Q: So why is knowing the half-life of a radioisotope IMPORTANT?

Ans:  In the case of the atomic bomb blast, it will help to determine whether the location has a level of radioactivity within safe limits.